SEK1 partially mediates arsenic trioxide-induced JNK activation in MEF cells. (A) SEK1^+/+ (upper panels) and SEK1^-/- cells (lower panels) were treated for 16 hours with a range of doses of As₂O₃ alone or in combination with 100 μM BSO. Immune complex kinase assays were performed to determine the effect of arsenic trioxide on JNK1 activity. A control blot directly comparing the activation of JNK by As₂O₃ in SEK1^+/+ and SEK1^-/- cells is also shown (B). Western blots of whole cell extracts from SEK1^+/+ and SEK1^-/- MEFs to detect total JNK1 expression levels are shown, as is a Western blot detailing the expression level of SEK1 (C).